1. Field of the Invention
The present invention relates to an ink jet printing apparatus that prints an image on a print medium by ejecting ink onto the print medium through nozzles formed in a print head, and in particular, to a printing apparatus using a print head having a plurality of relatively short chips which are arranged to increase the length of the print head and in each of which nozzles are arranged.
2. Description of the Related Art
Advantageously, ink jet printing apparatuses generate only low noise during printing because the apparatuses cause ink droplets to land on a print medium for printing. The ink jet printing apparatus also requires only low running costs owing to its capability of printing ordinary paper and the like without any special process. Furthermore, with the ink jet printing apparatus, using a plurality of color inks enable color images to be relatively easily formed. Moreover, densely arranging nozzles advantageously allows high-resolution images to be formed at a high speed. In particular, what is called a full-line printing apparatus is suitable for increasing the speed of the image forming operation; the full-line printing apparatus uses a long print head having a large number of nozzles arranged in a direction orthogonal to a direction in which print media are conveyed. The full-line printing apparatus may thus be used as an on-demand printing apparatus, the need for which is increasing. Accordingly, the full-line printing apparatus is thus gathering much attention.
The on-demand printing is expected to save labor instead of printing as much as several million copies as in the conventional printing of newspapers or magazines or performing printing at a very high speed, for example, printing one hundred thousand copies per hour. The full-line printing apparatus offers a lower print speed than conventional printers for offset printing or the like but eliminates the need to make printing plates, making it possible to save labor. The full-line printing apparatus further allows a wide variety of print matter in small quantities to be printed in a short time. Thus, the full-line printing apparatus is optimum for on-demand printing.
The full-line printing apparatus used for the on-demand printing is desired to print large-sized print media at a high resolution and a high speed. For example, the full-line printing apparatus needs to print at least 30 A3-sized print media per minute at a resolution of at least 600×600 dpi for monochromatic documents containing texts or the like or at a resolution of at least 1,200×1,200 dpi for full color images such as photographs.
The full-line ink jet printing apparatus is not only desired to print such large-sized print media but may also be used to print images taken with a digital camera or the like on L-sized media as in the case of conventional silver halide photography or on small print media such as postcards.
The full-line ink jet printing apparatus thus has excellent functions of dealing with print media of plural sizes and performing printing at a high speed. Accordingly, the full-line ink jet printing apparatus is expected to be widely used not only for business use but also for domestic use.
However, for the full-line printing apparatus, it is very difficult to form nozzles made up of ejection orifices, ink paths, or ejection energy generating elements, over a wide range equal to or greater than the print width of large-sized print medium without causing any defect. For example, a printing apparatus providing photographic outputs to large-sized sheets such as materials used in offices or the like needs about 14,000 ejection orifices (print width: about 280 mm) in order to print A3-sized print sheets at a high density of 1,200 dpi. It is very difficult to provide ejection energy generating elements corresponding to such a large number of ejection orifices without causing any defect, in connection with a manufacturing process. Thus, even if such nozzles can be manufactured, efficiency percentage is low and enormous manufacturing costs are required.
Thus, the full-line printing apparatus also uses a print head H such as the one shown in FIG. 1. The print head H is what is called a joint head formed by arranging a plurality of relatively short, inexpensive chips CH such as those used in serial printing apparatuses so that the chips are sequentially joined together to form an elongate head as shown in FIGS. 1 and 2.
In the joint head H, the plurality of chips CH are arranged along one direction. The chips CH located adjacent to each other in the chip arranging direction are shifted in the chip arranging direction and in a direction orthogonal to the chip arranging direction. The chips CH located adjacent to each other in the chip arranging direction have an overlapping portion (a joint portion or an overlapping portion).
However, with the joint head H, a print image is likely to be degraded in portions thereof corresponding to joints b and c of the joint head H owing to the configuration thereof. Specifically, the image is degraded if the direction in which the nozzles in the joint head H, shown in FIGS. 1 and 2, are arranged is inclined at a certain angle θ to a direction S orthogonal to a direction in which the print head H performs a scan operation relative to a print medium (with a full line head, a direction in which the print medium is conveyed). That is, if the print head is inclined as shown in FIG. 3, nozzle intervals in the head denoted by A, B, and C have values expressed by Formulae 1, 2, and 3. In the formulae, R denotes an inter-nozzle distance in the chips, Y denotes an inter-joint-chip distance, and θ(°) denotes the inclination of the joint head H.Nozzle interval A: R×COS(θ)  (Formula 1)Nozzle interval B: (R+Y×TAN(θ))×COS(θ)  (Formula 2)Nozzle interval C: (R−Y×TAN(θ))×COS(θ)  (Formula 3)
Specifically, determination may be made, as described below, of by what amount the nozzle intervals A, B, and C deviate from an inter-nozzle distance R (the nozzle interval obtained when the print head is located along the reference direction S (the inclination is 0°) if the print head is located under conditions described below.
It is assumed that the nozzles in the head shown in FIGS. 1 and 2 have a density of 600 dpi, and    inter-nozzle distance: R=42.3 μm,    inter-chip distance: Y=10 mm (=10,000 μm), and    head inclination: θ=0.05°. Then, the values of the nozzle intervals A, B, and C are determined in accordance with the formulae shown above. Then, the values obtained are compared with the inter-nozzle distance (R=42.3 μm).    Distance A: 42.29μ (almost no change)    Distance B: 51.03μ (an increase of 8.73 μm)    Distance C: 33.57μ (a decrease of 8.73 μm)
FIG. 19A shows a joint b including combinations (b1-b2) of nozzles having the nozzle interval B, shown in FIG. 3, and combinations (c1-c2) of nozzles having the nozzle interval C, shown in FIG. 3. As shown in FIG. 19A, in the joint b, four types of combinations (b1-b2) are possible for the nozzles having the nozzle interval B. Two types of combinations (c1-c2) are possible for the nozzles having the nozzle interval C. That is, in the joint b, the number of combinations (b1-b2) of the nozzles having the nozzle interval B is greater than that of combinations (c1-c2) of the nozzles having the nozzle interval C. Consequently, in the joint b, the number of areas printed by the nozzles having the nozzle interval B is larger than that of areas printed by the nozzles having the nozzle interval C.
FIG. 19B is a diagram showing an example of arrangement of dots printed by nozzles located in the joint b and the vicinity of the joint b when the print head H is tilted. In FIG. 19B, black circles denote dots printed by nozzles in a chip CH (N). White circles denote dots printed by nozzles in a CH(N−1). A method of printing dots corresponding to the joint b using the joint head H involves printing the dots so that the dots printed by the nozzles in the chip CH(N) alternate with the dots printed by the nozzles in the chip CH(N−1) array by array as shown in the figure.
In FIG. 19B, dots having a dot interval B′ are printed by the nozzles having the nozzle interval B. The other dots are printed by nozzles in the same chip, that is, the nozzles having the nozzle interval A. The nozzle interval A is almost equal to the nozzle interval R, corresponding to the non-tilted print head. Thus, the dots printed by the nozzles having the nozzle interval A are uniformly arranged. However, since the nozzle interval B is greater than the nozzle interval R, a blank is formed between the dots printed by the nozzles having the nozzle interval B, that is, the dots having the dot interval B′. This causes the vicinity of the joint b to be perceived as a white stripe.
FIG. 19C is a diagram showing another example of arrangement of the dots printed using the nozzles arranged in the joint b and the vicinity of the joint b. The method of printing the dots corresponding to the joint b differs between FIGS. 19B and 19C. In FIG. 19C, in the joint b, the dots printed by the nozzles in the chip CH(N) are staggered with respect to the dots printed by the nozzles in the chip CH(N−1).
In FIG. 19C, dots having a dot interval B′ are printed by the nozzles having the nozzle interval B. Dots having a dot interval C′ are printed by the nozzles having the nozzle interval C. The other dots are printed by nozzles in the same chip, that is, the nozzles having the nozzle interval A. In FIG. 19C, some of the dots are printed by the nozzles having the nozzle interval C, which is smaller than the nozzle interval R, corresponding to the non-tilted print head. However, as shown in FIG. 19A, in the joint b, the number of combinations (b1-b2) of the nozzles having the nozzle interval B is larger than that of combinations of the nozzles having the nozzle interval C. Consequently, in the joint b, the number of dots having the dot interval B′ is larger than that of dots having a dot interval C′. An area printed by the nozzles located in the joint b and the vicinity of the joint b is thus perceived as a white stripe.
FIG. 20A shows a joint c including combinations (b1-b2) of the nozzles having the nozzle interval B, shown in FIG. 3, and combinations (c1-c2) of the nozzles having the nozzle interval C, shown in FIG. 3. As shown in FIG. 20A, in the joint c, two types of combinations (b1-b2) are possible for the nozzles having the nozzle interval B. Four types of combinations (c1-c2) are possible for the nozzles having the nozzle interval C. That is, in the joint c, the number of combinations (c1-c2) of the nozzles having the nozzle interval C is larger than that of combinations (b1-b2) of the nozzles having the nozzle interval B. Consequently, in the joint c, the number of areas printed by the nozzles having the nozzle interval C is greater than that of areas printed by the nozzles having the nozzle interval B.
FIGS. 20B and 20C show the arrangement of dots printed by nozzles located in the vicinity of the joint c and the vicinity of the joint c when the print head H is tilted. Black circles denote dots printed by nozzles in a chip CH (N). White circles denote dots printed by nozzles in a CH(N+1). A method of printing dots corresponding to the joint c as shown in FIGS. 20B and 20C is the same as the method of printing dots corresponding to the joint b as shown in FIGS. 19B and 19C.
In FIG. 20B, dots overlap each other which are printed by the nozzles having the nozzle interval B, which is smaller than the nozzle interval R, corresponding to the non-tilted print head H. An area printed by the nozzles located in the joint c and the vicinity of the joint c is thus perceived as a black stripe.
In FIG. 20C, some of the dots are printed by the nozzles having the nozzle interval B, which is larger than the nozzle interval R, corresponding to the non-tilted print head. However, as shown in FIG. 20A, in the joint c, the number of combinations (c1-c2) of the nozzles having the nozzle interval C is larger than that of combinations of the nozzles having the nozzle interval B. Consequently, an area printed by nozzles located in the joint c and the vicinity of the joint c, the number of dots having the dot interval C′ is larger than that of dots having the dot interval B′. The area printed by nozzles located in the joint c and the vicinity of the joint c is thus perceived as a black stripe. As described above, tilted joint head may result in a white or black stripe in the area printed by joints of the print head, degrading the quality of recorded images.